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Try one problemGiven a matrix of 0s and 1s, return a matrix of the same size where each cell holds the distance to the nearest 0, with distance measured between 4-directionally adjacent cells.
▸1dist = grid; mark 1-cells unknown2queue = all 0-cells // multi-source seed, distance 03ring = 04while queue not empty:5 ring++6 for each cell in this frontier:7 for each unknown neighbour:8 dist = ring; enqueue it9 queue = next frontier10return dist
line 1The 01-Matrix: replace every cell with its distance to the NEAREST 0 (moving up/down/left/right). A naive idea — run a BFS from each 1 — is wasteful. Instead, flip it around: do ONE BFS that starts from ALL the zeros at once. The wavefront from the zeros reaches each 1 in increasing distance, so the first time a 1 is touched, it is touched by its closest zero.